This invention relates to a method of embossing in convolutely wound rolls to avoid nesting and the product resulting therefrom. Exemplary of the products produced according to the invention are household toweling and toilet tissue.
For many years, the problem of pattern "nesting" has been recognized as being contrary to the objective of obtaining maximum bulk and roll diameter for a specific sheet count and roll footage. The phenomenon of nesting applies to all roll products produced heretofore and vis-a-vis, sanitary roll products like toweling or bathroom tissue, many different patterns and techniques have been developed in order to avoid nesting. The problem still persists despite development of patterns that have undulating "sine waves", randomized patterns, different sized elements, and different sized pattern repeats. This difficulty has frustrated the achievement of the desired bulk which has been considered advantageous not only from the aesthetic point but also has provided certain manufacturing efficiencies.
For the sake of description, pattern repeat is the same as pattern sketch size and represents a specific length of pattern that is unique onto itself albeit it may contain the plurality of elements which are identical. For example, a plurality of identical elements can be laid out and subsequently engraved on embossing rolls, such that in a given "sketch repeat", elements that are aligned in both the MD ("machine direction") and CD ("cross machine directions") but spaced apart 1/8" would have a sketch repeat of 1/8".
In other arrangements, the same plurality of elements can be skewed along diagonal or undulating "sine wave" lines such that the repeat can be practically any length, and if they were laid out in a sine wave pattern, the sketch repeat would represent the pitch length of the sine curve as it progresses from a central reference to one side, crosses at inflection, continues past the mid point and finally returns to the same center line. This would be the pitch length of a sine wave having a plurality of common elements.
Pursuing the solution to this problem has thus far been a never-ending one. For example, U.S. Pat. No. 4,181,068 recognizes the problem and states "in order to preserve the desired structure and absorption characteristics in the paper towels, it is desirable to prevent the bulk characteristics of the towels from being deformed". This patent describes elements placed symmetrically about a center line that forms a helix of about 5 degrees relative to the MD. However this approach does not avoid the twin problems of embossment nesting and embossment breakdown.
There are numerous examples of commercially available products wherein the elements themselves or their placement have been randomized in order to prevent nesting, but as described more completely herein, these attempts have fallen short of a true solution to the problem. To understand why a truly non-nested convolutely wound roll has not been developed, a brief history of embossing is set forth.
Referring to the time frame of the 1950's and early 1960's, it was common practice to emboss with a male or female engraved steel roll bearing against a paper roll which ultimately wore into the same pattern and formed a close full contact nip between said rolls. Because paper rolls were subject to wear, they would, at the outset, be sized approximately 0.060" over the true pitch diameter of the steel engraved roll. The normal operating range was from 0.060" over the diameter of the engraved roll to minus 0.060" under sized. This limited operating range dictated expensive roll change rather frequently. In addition, and because of heavy nip pressures there was theory that due to the resiliency in the paper filling, a small upward deformation was always present just before the nip. When speeds increased beyond the limit of paper resiliency, this deformation or hump passed through the nip, setting up violent chattering of the roll and destruction of the pattern formed in the paper filling. In essence, a paper-steel combination presented a speed limitation in production. It should also be noted that because of the difficulty of running the pattern into the paper roll, pattern depth was generally limited to a range of 0.030"-0.035" to avoid excessive run in time and "scrubbing" of the pattern by interaction between the steel and paper rolls--this being a function of element shape, the angle of the element side wall and numerous other factors.
To avoid speed and replacement problems, matched steel engraved rolls entered the scene. In effect, the first roll is made from a mated tool or die (male or female) and substantial mechanical pressures are used to develop the pattern on the surface of the first steel roll, sometimes in combination with chemical etching. When the first or "conventional" roll was made, the second roll was generated by carefully controlling the chemical etching process and in essence, eating minute amounts of metal away until the surface of the second roll would perfectly mate with the first roll. If the first roll were male or cameo, the second roll would be female or intaglio.
With the development of steel-to-steel engraved rolls and their use in production, normally just ahead of a roll rewinder, there was still concern with substantially incompressible paper wads or other foreign matter being carried by the web through the nip causing instantaneous deflection, instantaneous recovery and damage when mating roll surfaces contacted. To avoid this, steel-to-steel rolls are commonly run with a minimum 0.010" clearance, but since the paper itself, especially one-ply is normally in the range of 0.005", the use of steel-to-steel rolls prompted development of deeper patterns--often in the range of from 0.035" to as high as 0.070" or above.
Recognizing the fragile nature of single ply tissue, or even two-ply tissue and toweling, and especially in view of the stock used, for example ground wood or short fibers, etc., the general trend toward larger elements was somewhat counter productive depending on the ability of the paper webs to sustain embossments during the wind up process. With larger elements, steeper element sidewalls became necessary, and in the female roll (first or second roll), this resulted in deeper grooves which can easily build up and pack with paper dust forcing periodic shutdown and arduous roll cleanup.
The larger elements also were more prone to failure when they were wound in a convolutely-wound roll simply because they would not sustain the next one or two convolutions of paper. At this point it is noted that a certain amount of web tension was necessary between an embosser and the rewound roll in order to avoid excessive wrinkles, and this necessary tension caused deeper, larger elements to collapse under the influence of outer convolutions.
Recognizing this problem, it might be thought that if an embossed web would perfectly nest throughout the wind, the upper embossment would be nested within the underlying embossment and thus be protected from damage by subsequent convolutions of wound product. However, to produce this effect, relatively small pattern sketches would to be used at the beginning of the wind and the "sketch repeat" would have to become progressively larger to account for diameter buildup. Thus, the circumference of a matched steel roll would have to be greater than the total footage in the wound roll, for example, a typical product, 187.5 lineal feet would dictate a roll diameter of 59.68" (almost six feet)--a totally unacceptable solution.
The other solution was to randomize pattern with different element shapes, or arrange a plurality of similar elements in randomized layout. However, according to present practice, and reference newly issued U.S. Pat. No. 4,181,068 it will be seen that because of the engraving process and the tooling thus far developed, pattern repeats currently used are limited in the range from as small as 0.0625" to as high as 5.0", and these repeats cannot be made to avoid sequential nesting and non-nesting throughout the normal roll buildup. Relative to single or two-ply tissue or toweling, the phenomenon of nesting/non-nesting throughout the roll buildup occurs. Where the larger, deeper elements are non-nested, failure occurs in zones adjacent to the nested portion for several reasons--the significant one being the continued advancement of any given pattern as the roll is wound and its tendency to "climb out of" the nested condition.
In recent years, and in recognition of the inability to generate maximum bulk, there has been a substantial trend toward laminated two or three ply toweling produced according to U.S. Pat. Nos. 3,337,388, 3,414,459, 3,961,119, and co-owned U.S. Pat. No. 3,867,225. These laminating techniques are effective on heavier weight two-ply toweling, but are not well adapted to light weight single or two-ply tissue products because the adhesive migrates rapidly through one or both plies at the point of application, thus fouling the co-acting embossing rolls. The invention does not involve laminating techniques and thus avoids the maintenance problem as well as the cost of adhesive.
The invention was prompted by the phenomenon of nesting/non-nesting and the resultant destruction of embossments in the wound roll first appeared in about 1977 in rolls produced in a Canadian mill. The toilet tissue web was embossed with a pattern using sequential and series male-female elements each about 0.1875".times.0.1875" (3/16") with a sketch repeat of 0.375" (3/18"). Collapsing of embossments and degradation of the product was very apparent, but the failure of embossments (consisting of variable pluralities) occurred at various distances from the core in an unpredictable and non-related fashion. Despite efforts to control tension more accurately, the random failure of a variable plurality of embossments continued. This led to the investigation set forth hereinafter with the ultimate discovery of the inventive solution to the problem.